1. Field of the Invention
The invention generally relates to a frequency error and phase error correction and/or detection for use in units or subunits of data communication systems, and in particular for use in WLAN (Wireless Local Area Network) receivers.
2. Description of the Related Art
In a communication system, it is important to synchronize the receiver to the transmitter so that messages can successfully be exchanged between the transmitter and the receiver, which is of particular importance for modern data communication systems such as wireless local area networks.
A wireless local area network is a flexible data communication system implemented as an extension to or as an alternative for a wired LAN. WLAN systems transmit and receive data over the air using radio frequency or infrared technology to minimize the need for wired connections. Thus, WLAN systems combine data connectivity with user mobility.
Most WLAN systems use spread spectrum technology, a wide-band radio frequency technique developed for use in reliable and secure communication systems. The spread spectrum technology is designed to trade-off band-width efficiency for reliability, integrity and security. Two types of spread spectrum radio systems are frequently used: frequency hopping and direct sequence systems.
The standard defining and governing of wireless local area networks that operate in the 2.4 GHz spectrum is the IEEE 802.11 standard. To allow higher data rate transmissions, the standard was extended to the 802.11b standard, which allows data rates of 5.5 and 11 Mbps in the 2.4 GHz spectrum. This extension is backwards compatible.
The standards for WLAN systems using direct sequence spread spectrum techniques employ a training preamble to train the receiver to the transmitter. Each transmitted data message comprises an initial training preamble followed by a data field. The preamble includes a sync field to ensure that the receiver can perform the necessary operations for synchronization. For the preamble length, two options have been defined, namely a long and a short preamble. All compliant 802.11b systems have to support the long preamble. The short preamble option is provided in the standard to improve the efficiency of the networks throughput when transmitting special data such as voice and video. The synchronization field of a preamble consists of 128 bits for a long preamble and 56 bits for a short preamble.
A receiver detects the synchronization symbols and aligns the receiver's internal clock to the symbols in the synchronization field in order to establish a fixed reference timeframe with which to interpret the fields in the transmission frame structure following the preamble. The preamble, including the synchronization field, is transmitted at the start of every message (data packet).
FIG. 1 shows a block diagram of a prior art WLAN receiver 100. Via one or more antennae 110 the receiver receives a data stream from a WLAN transmitter and feeds the antenna output to a radio front end unit 120. In the radio front end unit the received data signals are preprocessed and handed over to the synchronization unit 130. After synchronizing the received data signals the synchronized data signals are handed over to the digital signal processing unit 140 for further digital signaling processing. The antenna selection is done by the antenna diversity controller or finite state machine.
Its purpose is to measure at the beginning of the preamble which antenna delivers the strongest signal. This antenna will be the receive antenna for the frame. After selecting the antenna the preamble is detected by a preamble detection unit that scans the incoming data stream for a preamble while the receiver is in the receive mode. Its purpose is to detect a preamble and to determine whether a short or a long preamble is being received. It will also determine the boundaries between consecutive Barker symbols such that the following processing blocks can adjust their processing schedule accordingly. Finally, it will deliver an initial frequency error estimate that will be used in the frequency error correction module for an initial frequency error correction. Moreover, a synchronization unit performs a phase error correction for the remaining phase drift after the frequency error correction.
In WLAN systems, as well as in other spread spectrum communication systems, the signal-experiences a variety of distortions on its way from the transmitter to the receiver, which result in frequency an phase changes of the signal. Furthermore, a frequency or phase error may result from a frequency or phase offset of the radio frequency oscillators at the transmitter and the receiver. Moreover, the oscillators may provide different frequencies due to manufacturing imperfections, different temperatures, etc. which result in a frequency drift off the base band signal. Therefore, the synchronization unit performs a frequency error correction and a phase error correction.
Conventional frequency error correction modules in receivers still have a number of disadvantages. One problem is that frequency error correction modules need to perform a time-consuming number of iterated steps to achieve frequency synchronization.
First stage frequency offset corrector mostly does only course compensation. Nevertheless a residual frequency offset remains. A frequency-offset distortion can be viewed as a constantly changing phase-drift, which rotates in the complex plane with the frequency offset. This phase-drift causes a constant phase estimation error in the phase error correction. This error depends equally on the frequency-offset value and the group delay of a phase error correction low-pass filter. This results in performance degradation.
Conventional phase error correctors are designed to compensate for a static or slow varying phase-offset. To be stable against noise and distortions they always need to average over a certain number of phase-estimates to filter out those variations. This low-pass filter employs a system specific delay of the averaged phase estimates against the actual incoming signal to be phase corrected. If the phase variations are slow enough this effect does not matter but for quicker phase variations a residual phase-offset remains even after correction. In a following coherent decoding/demodulation part of the receiver this residual offset causes performance degradation.
Although different circuits and methods are known in the art to speed up the acquisition time of phase locked loops and to reduce frequency-offsets they often have been insufficient for compensating for the residual frequency offset and the actual phase offset. Furthermore, the conventional techniques often cannot provide a fast enough convergence and strong enough noise resistance.
Due to the wide range of different tasks the synchronization circuits in existing WLAN receivers are very complex. As the digital signal processing functions need a plurality of functional units the circuits are highly involved. Therefore the costs of circuit development and manufacturing are high.